Originally the magnetic resonance phenomena was utilized to study the molecular structural of organic molecules in-vitro. This was in the field of NMR spectroscopy. Typically the magnetic resonance spectrometers utilized for spectroscopy were designed to accommodate relatively small samples of the substances being studied. In the relatively recent past, however, magnetic resonance has been used for medical diagnostic imaging. Using magnetic resonance methods tomographic images have been obtained of the interior of live human subjects. Such images generally detect parameters associated with "nuclear spins". Nuclear spins are typically hydrogen protons associated with water in tissue. However, the NMR techniques have also been extended to in-vivo spectroscopy of such elements as phosphorous, sodium and carbon, for example. This provides researchers with efficient tools for the first time enabling the study of chemical processes in vivo. The use of magnetic resonance to produce images and to produce spectroscopic studies of the human body has necessitated the use of specifically designed radio frequency coils or probes.
As is well known, magnetic resonance occurs in nuclei that have an odd number of protons and/or neutrons. Since protons and neutrons spin about the axis, each such nuclei that has an odd number of protons or neutrons exhibits magnetic moment. When such nuclei ("spins") are placed in a large static homogeneous magnetic field Bo, to a statistical majority of the nuclear magnetic moments align with the field to produce a macroscopic magnetisation in the direction of the large static magnetic field. Under the influence of the large magnetic field Bo, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, fo is also referred to as the Larmor frequency and is given by the equation: fo=.gamma.B/2.pi.;
wherein:
.gamma. is the gyromagnetic ratio (a constant for each NMR isotope); PA0 B is the magnetic field (Bo plus other fields) acting upon the nuclear spins; and PA0 .pi. is the well known constant 3.1416+. PA0 a first loop, PA0 said first loop comprising: first and second longitudinal substantially parallel conductors, PA0 a pair of transverse conductors coupled to the first and second longitudinal conductors at opposite ends thereof to form said first loop, PA0 first capacitor means in the first loop for tuning and matching the first loop to a first desired resonant frequency and to the system impedance, PA0 second and third loops comprising: PA0 a third longitudinal conductor connected between substantially equal voltage points on said pair of transverse conductors to thereby provide said second and third loops, and PA0 second capacitor means for tuning and matching said second and third loops to a second desired resonant frequency and to the system impedance.
From this equation it is apparent that the resonant frequency is dependent on both the strength of the magnetic field and the particular isotopes included in the sample being studied.
The density of the spins or the spectrographic response of the spins is determined by perturbing or "tipping" the spins into a transverse plane or such that a component of the spins appear in the transverse plane, i.e. the plane normal to the axis of the large static homogeneous magnetic field. The spins are tipped by the application of radio frequency (RF) magnetic fields that oscillate at or near the Larmor frequency. The RF magnetic fields are applied orthogonal to the direction of magnetization of the large static magnetic field and by means of radio frequency pulses transmitted through RF coils connected to radio frequency transmitting apparatus. The spins so tipped tend to lose the energy that was supplied by the RF field and revert to the former alignment. As they revert they generate RF signals either in special receiving coils or in the same coils which transmitted the RF pulses that caused the spins to tip. Naturally, the signals which are known as free induction decay (FID) signals are very small and everything possible is done to enhance the signal-to-noise (SNR) ratio of those signals.
When the whole body is imaged to obtain a tomographic section of the whole body then what are known as body coils are used for both transmitting RF pulses and receiving the RF signals. The body coils are large coils that surround the patient's entirely.
Many times only portions of the body are imaged. For example, if the image is done on the spine or if the image is done one of the limbs, then what are known as surface coils are used. The surface coils are used primarily to detect the FID signals responsive to RF pulses applied through a body coil. The advantage of surface coils is that they can be applied or situated immediately adjacent or juxtaposed to the portion of the body being imaged. This proximate positioning, of course, increases the signal-to-noise ratio.
Another method of increasing the signal-to-noise ratio is by through the use of what are known as quadrature coils. Quadrature coils have inductive receiving portions separated by 90 degrees. Such coils can be thought of as utilizing a rotating field wherein the angular rotation of the field can be set to synchronize with the precessing frequency.
When data from protons are being acquired, then a resonant or Larmor frequency is used based on the Larmor frequency equation discussed above. For example in a 1.9 Tesla field, the magnetic resonance frequency of protons is 81 MHz. For spectroscopic studies different resonance frequencies are required to resonate such elements as phosphorous, sodium or carbon. The resonant frequencies for those elements are 33 MHz for phosphorous and approximately 21 MHz for both sodium and carbon.
In the past different coils have been used for detecting the resonant frequencies of the different elements. However, recently a patent (U.S. Pat. No. 4,691,163) was issued to the Assignee of this Application covering a surface coil capable of resonating at more than one frequency. Such a feature is very desirable in magnetic resonance systems because among other things the same coil can be used both for imaging and for spectroscopic studies.
An object of this invention is to provide RF coils that can be used not only for detecting resonance at more than one frequency, but also for effectively detecting signals in either quadrature or linear modes.